1. Field of the Invention
The invention relates generally to devices for terminating transmission lines and specifically for terminating a transmission line to reduce electromagnetic interference in electronic systems and networks.
2. Relevant Background
Whenever an electric charge is accelerated, electromagnetic waves are generated. Typical electric and magnetic fields in electronic circuits are generated by current pulses propagating along a path or a loop within the circuit. Each current pulse that propagates along the path creates a magnetic field perpendicular to the plane of the current path. The resulting voltage drop along the path creates an electric field opposite to the propagation direction and within the same current plane. Most common current paths within a personal computer consist of I/O cables, printed circuit board (PCB) signal traces, power supply cables, and power-to-ground loops. These paths can act as antennae, radiating electric and magnetic fields that cause EMI by interacting with other signals. The magnitude of EMI is a function of several characteristics of the transmitted signal, such as its frequency, duty cycle, edge rate, and voltage swing (amplitude). This EMI may result in erroneous transmission of data, lost data, or a reduction in the amount of acceptable noise for that system.
As the computer market evolves, increasingly higher-speed data processing and transmission technologies are being developed. Electronic components and circuits, such as microprocessors, operate at increasingly higher frequencies and lower voltages and are increasingly more susceptible to electromagnetic interference (EMI). Unfortunately, nearly any computer system has the potential for causing EMI during operation.
Another source of EMI, aside from I/O cables, PCB signal traces, power supply cables, and power-to-ground loops, can arise when high-speed data is transmitted to the pins of an unterminated connector. In this situation, the open pins act as small antennae that radiate the transmitted signals. These open pins have been observed to generate up to 10 decibels or more of EMI. The EMI can interfere with other components within the computer system as well as other susceptible electronic systems that may be nearby. Thus, whether the open pins reside within or outside of a computer system housing, it is desirable, and in some situations necessary, to reduce these emissions to acceptable levels.
In the prior art, various techniques are recommended to reduce EMI in data transmission lines. See “Characteristics and Measurement Techniques of the Spectral Content of Signals Generated by High-Performance ICs”, Fairchild Semiconductor Application Note, June 1992 (AN-831), revised November 1999 (AN010998). One technique known as the parallel termination scheme matches the effective impedance of the transmission line with a resistor coupled in parallel. Another technique known as the series termination scheme places a resistor in series with the output driver and the transmission line. The resistor value is selected such that when added to the integrated circuit (IC) output resistance, the total equals the effective impedance of the transmission line. This effectively forms a voltage divider with the transmission line producing a half-voltage level at the source which doubles upon reflection at the end of the line. These techniques are applicable to distributed or point-to-point data transmissions, respectively, but do not address the issue of open connector pins at the end of the transmission cable.
Similarly, other components such as ferrite cores and beads, feed through capacitors, connector shields, gaskets, and conductive tapes can all prevent unwanted EMI signals, as known in the art. These techniques are not suitable, however, for use on connector pins because the components would interfere with mating the pins to a corresponding connector. It is therefore desirable to provide a device for reducing, and even eliminating, EMI propagated by data being transmitted to an unterminated portion of a connector.